ftp: Reenable support for RFC 2428

To avoid that IPv6 enabled clients use extended passive and extended port
commands on IPv4 connections, support for these commands is limited to
IPv6 connections. This is necessary as using these commands prevents
ftp data connections from being redirected to pools.

Target: trunk
Require-notes: yes
Require-book: yes
Acked-by: Karsten Schwank <karsten.schwank@desy.de>
Patch: http://rb.dcache.org/r/6525/